The moderator recently solicited for content so I offer my modest project idea and drawing.
I recently finished drawings for a four cylinder model engine. It would run on propane and have electronic ignition. The intent was to draw an engine that my antiquated machines could handle and for an engine that would run and be reliable and simple. I'm still not sure I can do it but it keeps me inspired.
The crank would be supported on the ends only (twin ball bearings each end) and the full-diameter crank weights would serve as a portion of the flywheel mass.
The pump in the drawing is a dual-chamber gear pump; one side is for raw water and the other for engine oil.
The external timing gears may be noisy but past experience with belts wasn't successful.
Alternately, the one I build may be the same design in a three cylinder version with a slightly higher compression ratio.